Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO2++ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO2++ distribution declining steadily from 5.6 cm−3 at 160 km to below 1 cm−3 above 200 km. The dominant CO2++ production mechanisms are double photoionization of CO2 below 190 km and single photoionization of CO2+ at higher altitudes; CO2++ destruction is dominated by natural dissociation, but reactions with atmospheric CO2 and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger solar control of the CO2++ density than of the CO2+ density .

The profiles of the Martian dayside ionosphere can be used to derive the 130km neutral atmospheric densities, which can also be obtained from the Mars Climate Database (MCD) and the spacecraft aerobraking observations. In this research, the method to calculate 130km neutral densities through ionosphere observations is explained, and three long-period 130km neutral densities data sets at northern high-latitudes (Lat >60°) are acquired through the ionospheric data measured by MGS Radio Occultation Experiment. The calculated 130km neutral density data, along with the 130km density data from the aerobraking observations of MGS and ODY in the northern high-latitudes, are compared with the MCD outputs with the same latitude, longitude, altitude, solar latitude and local time. It is found that both the 130km density data derived from ionospheric profiles and aerobraking observations show similar seasonal variations as the MCD data. With a negative shift of ~2x1010 cm -3, the corrected 130km neutral densities derived from the MCD v4.3 are consistent with those obtained from two different observations. This result means that 1) the method to derive 130km neutral densities through ionospheric profiles is effective, 2) the MCD v4.3 data sets generally overestimate the 130km neutral density at high-latitudes, and 3) the neutral density observations from MGS Radio Science Experiment can be used to calibrate the new atmospheric model of Mars.